Naomi Watts's Post-Birth Fashion Tip: 'Solid Undergarments'

New mom Naomi Watts says juggling work and motherhood is hard – but looking svelte post-pregnancy is easy if you know how.

The 39-year-old actress and new mom, who gave birth to son Alexander Pete less than three months ago, showed off a trim and toned physique on Monday night while hosting a Calvin Klein event in London

Her secret? "I've got a lot of solid undergarments," she giggled to PEOPLE as she looked down at her figure-hugging black Calvin Klein sheath dress.

And of course a fabulous designer dress helps as well. "Good classic lines can also help cheer up an out of shape body," she said with a nod to her co-host, Calvin Klein designer Francisco Costa.

Watts enjoyed drinks and canapés with guests such as Bianca Jagger and Mary-Kate Olsen while viewing the designer's 2008 spring/summer collection, part of a live art installation by artist Martin Creed.

The actress, who is in London to promote her film Eastern Promises, which will open the London Film Festival on Wednesday, attended the fashion soiree without partner Liev Schreiber. Last week the family spent time in Lithuania, where Schreiber is filming his latest movie, Defiance.

Mixing a career with family life, she told PEOPLE is ultimately "hard work." But it seems the actress wouldn't change a thing. Asked about how she likes motherhood, she beamed with delight and simply said: "Love it, love it!"